Aminophylline, nitroglycerin, hydralazine, and some of the prostaglandins are a few examples of a large group of therapeutically important or potentially important smooth muscle relaxing agents. The modes of action of these agents have not, as yet, been clearly established. It would appear that alterations in calcium and cyclic AMP metabolism are somehow involved. Unresolved questions regarding the manner in which these spasmolytic agents modify the mechanical activity of smooth muscle fibers will be investigated by exloring their effects: 1) on the rate of relaxation of vascular and intestinal smooth muscle preparations that have been induced to undergo relaxation by various experimental interventions, 2) on dose-response relationships (agonist concentration vs mechanical response) in smooth mucle fibers that are to be analyzed and interpreted by a newly coneived method, and 3) on the intracellular quantity and distribution of calcium estimated by a histochemical method. It is anticipated that the information obtained will help pinpoint the modes and sites of action of the spasmolytic agents in the smooth muscle fiber, increase our understanding of the normal series of cellular reations that occur during the development of a mechanical response, help define the role of cyclic AMP in the relaxation process, and possibly reveal which of the relaxing agents may be used in combination to produce a more powerful peripheral vasodilator or bronchiodilator action.